There aren't enough blacks in the CSA to pull off a successful Revolution.
Actually, there are. In many places slaves constituted over 50% of the population. The problem was not numbers but an inability to organize and communicate, as well as the strenuous efforts by the slave-owning class to prevent slaves from becoming literate.
Now if the war ends as a Confederate victory in late 1863 or 1864, the situation will change, because you will have had hundreds of thousands of slaves freed by the Union army, with idealistic volunteers teaching many to read and write. Most importantly, tens of thousands joined the Union army, learned to use firearms, and were taught military discipline.
They might succeed in seceding if the CSA is weak enough and they get Northern backing.
They may not get Northern backing, though. Even at the height of the war, support for abolition was lukewarm in many parts of the Union outside of New England. Many classes, including new immigrants from Europe, saw free blacks as nothing but competitors for low wage jobs. In the event of a large-scale slave revolt in the South, it would not surprise me if the United States sat on the sidelines.